This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to MRI using steady state free precession (SSFP) with image artifact reduction.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Magnetic resonance (MR) imaging is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During a MR experiment, each nuclear spin responds to four different effects—precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MR experiments, a combination of these effects occurs periodically.
Refocused steady-state free precession (SSFP) sequences have recently gained popularity in magnetic resonance imaging, due to improved gradient hardware. SSFP imaging provides high signal and good contrast in short scan times. However, in regions of high local magnetic field variations, SSFP images often suffer from characteristic bands of signal loss, or “banding artifact”.
The banding artifact becomes more pronounced as the main magnetic field strength B0 and any inhomogeniety therein is increased, due to the increased off-resonance precession per sequence repetition time TR. Likewise, as the sequence repetition time TR is increased, any off-resonance banding artifact will become more pronounced due to the increased off-resonance precession per TR. There is currently considerable interest in using SSFP sequences at both higher main field strengths and for higher resolution images, which usually require longer TRs. However, both of these changes will increase the severity of the banding artifact. A method for reducing or eliminating off-resonance banding at higher field strengths or longer TRs would clearly be useful.
Several methods have been proposed for reducing the banding artifact in SSFP imaging by combining multiple phase-cycled image acquisitions in various ways. These include a maximum-intensity combination method and a simple complex-sum combination method. The effectiveness at removing banding artifacts varies with number of acquisitions combined, tissue parameters T1 and T2, and tip angle. As multiple acquisitions of the same image must be performed, the SNR efficiency of each of these techniques suffers when compared with the SNR efficiency of a single SSFP image acquisition.